The invention concerns a process and a machine for surface sizing flexible parts having a porous surface, such as fabric parts, foam parts etc, with a water-based adhesive.
At the present time, flexible parts made of fabric and/or foam are sized either with adhesives of the hot melt type or with adhesives in a volatile organic solvent of which the composition may be easily chosen in order to prevent any penetration of the adhesive into the thickness of the parts.
Indeed, when it is desired to size a flexible part having a porous surface, it is important to prevent, or in any case to minimize, penetration of the adhesive into the thickness of the part in order to prevent, on the one hand, a boarding effect (i.e. a hardening of the superficial thickness of the surface affecting its flexibility qualities) and, on the other hand, a waste of adhesive (only the adhesive deposited on the surface providing subsequent adhesion). Such is, for example and in particular, the case when seat covers and seat cushions are sized for their relative assembly by gluing.
Now, the volatile organic solvents used (acetone, toluene, methanol etc) are toxic and harmful to the environment and human health.
In addition, adhesives are known without a volatile organic solvent such as so-called "water-based adhesives". These water-based adhesives are adhesive compositions formed of a base of one or more synthetic or natural adhesives (polyurethanes, elastomers, thermoplastic polymers, acrylics, synthetic resins etc) dispersed in water. Nevertheless, these water-based adhesives have a much longer drying time and are difficult to transfer on account of their tendency to adhere to the walls of pipes, to precipitate and to form plugs. They are however fluid and tend inevitably to penetrate into the pores of the surfaces to which they are applied.
It should be noted that, in these water-based adhesives, unlike solvent-based adhesives, most of the dry solids which are proportionally much higher in quantity (of the order of 50%), are not dissolved but are in suspension, water only having the function of liquefying the composition and acting as a vehicle for the adhesive bases. As a consequence, the techniques used for sizing with solvent-based adhesives cannot be transposed to water-based adhesives.
So-called "two-component" water-based adhesives are also known, namely those associated with a second composition sprayed simultaneously and intended to accelerate the polymerization of the adhesive. However, this technique then requires the use of two costly application devices (two atomizing nozzles), which accordingly increases the risk of blockage as well as investment and running costs. This solution is therefore not compatible with continuous industrial production. Moreover, the said second composition based on ammonia or salts has a strong odor, is an irritant and attacks the skin and respiratory tracts.
In all cases, water inevitably penetrates into the pores, producing residual moisture which is harmful and undesirable in the part.